Endoscopes are conventionally used at medical treatment facilities for observing or providing treatment within the body of a patient. The objective lens currently proposed for use in this type of endoscope has a back-focus of zero, with the image-side surface of the final lens element in contact with an end surface of a fiber optical bundle of an image guide, as in Japanese Laid-Open Patent Application H02-69710. However, in such a case, there is concern that the total length of the objective lens may become too long, making the endoscope difficult to operate. Therefore, an objective lens disclosed in, for example, Japanese Laid-Open Patent Application H02-176612 has been proposed as an objective lens with a short focus adjustment space. Further, the objective lens described in, for example, Japanese Laid-Open Patent Application H09-68647 discloses a compact, three lens element construction.
However, since the objective lens of the above-mentioned Japanese Laid-Open Patent Application H02-176612 has a comparatively large positive refractive power in the third lens element, an aspheric surface must be introduced in order to correct various aberrations such as coma and astigmatism, making it difficult to achieve a high precision due to problems in processing. On the other hand, with the objective lens described in the above-mentioned Japanese Laid-Open Patent Application H09-68647, correction of spherical aberration is not always sufficient due to the placement of the diaphragm and the amount of refractive power of the first lens element.